1. Field of the Invention
The invention relates to a sigma-delta modulator for converting broadband digital input signals.
2. Description of the Related Art
In digital-to-analog converters, as used, for instance, in digital radio communication systems, a digital input signal having 2N signal statuses and a fixed sampling frequency fa is customarily converted into an analog signal needing to tally as closely as possible with the digital signal in the −fa/2 to +fa/2 frequency range.
The number of signal statuses requiring to be implemented by analog circuitry poses a substantial problem especially when the bit widths N are large. The digital signal is for this reason interpolated by digital filters and what are termed sigma-delta modulators are employed in the digital-to-analog converters which significantly reduce the digital signal's bit width with an increased sampling frequency and transform the quantizing noise increased thereby into previously unused frequency ranges. Structures of sigma-delta modulators achieving forming of the noise signal by higher-order IIR (Infinite Impulse Response) filters are particularly efficient here.
A digital-to-analog converter employing an IIR filter as the interpolation element and one or more sigma-delta modulators for converting the interpolated signals is described in, for example, U.S. Pat. No. 5 786 779.
A cascaded sigma-delta modulator for a digital-to-analog converter is furthermore presented in DE 197 22 434 C1. S. R. Norswothy, R. Schreier, G. Temes: “Delta-Sigma Data Converters, Theory, Design and Simulation”, IEEE Press 1997, ISBN 0-7803-1045-4, contains a detailed description of the design of sigma-delta modulators and their mode of operation.
Two approaches are now taken in the sigma-delta modulators to achieve a noise forming:
According to a first approach, higher-order feedback loops are used which permit a reduction to up to two signal statuses (1-bit signaling technique). However, upward of order-3 noise forming these can lead to possible instabilities in the case of high input signals; overshooting of the value range of internal status memories very easily occurs. To counter this, an amplitude-reduced input signal and status memories having clipping characteristics are used in practice as a result of which empirically ascertainable circuit stability can be achieved.
According to a second approach, first- and/or second-order cascaded structures are used which, being of multi-stage design, have stable operating characteristics.
The sigma-delta modulator for converting digital input signals x(k) can include a first feedback loop of a spectrally formed output signal y(k) of the sigma-delta modulator and a second feedback loop of a spectrally formed differential signal e(k) obtained from an intermediate signal u(k) and from the output signal y(k), with the intermediate signal u(k) being the differential signal of the input signal x(k) and the total signal r(k) of the first and second feedback loops, with a quantizer determining the output signal y(k) based on the intermediate signal u(k), and with k being the discrete independent time variable.
A cascaded sigma-delta modulator having the advantages, on the one hand, of a cascaded approach's stability in terms of operating characteristics and easier implementability and, on the other hand, the advantages of a small number of stages of a higher-order feedback loop has been described in DE 199 37 246 A1. The number of signal statuses can be reduced to up to two—corresponding to 1 bit—by incorporating an additional logic. Complex clipping circuitry is eliminated without impairing circuit stability. Owing to the circuit's modularity, an existing structure of an i-th order sigma-delta modulator can be expanded into an i+1-th order circuit by simply an additional logic stage.
Through the cascading of several first-order modulators the quantizing noise in the low frequency range is evaluated particularly intensely in the target function used. The quantized signal needs to exhibit extreme variations in the time domain in order to transform the quantizing noise into the frequency range. The possibility for this is not provided in the case of, for instance, a two-stage signal and is suppressed by the structure according to DE 199 37 246 A1. Consequently, although noise forming has a higher order as a result of cascading, there is only a limited increase in the frequency range with a specific signal-to-noise ratio at a specified measuring bandwidth, especially in the case of a desired relatively low signal-to-noise ratio.